Recently, an increasing desire for better fuel economy of vehicles has demanded tire rubber compositions excellent in fuel economy. A known method for improving the fuel economy is to modify styrene butadiene rubber or butadiene rubber so as to promote dispersion of filler. An exemplary proposed modified rubber for silica formulation is a rubber having a terminal modified with an alkoxysilane containing a nitrogen functional group, and an exemplary proposed modified rubber for carbon black formulation is a terminal-modified butadiene rubber which is coupled with tin. Additionally, mercapto group-containing silane coupling agents disclosed in Patent Literature 1 are known to be highly reactive with silica and promote dispersion of silica.
However, all the above-mentioned techniques for improving the fuel economy are effective for styrene butadiene rubber or butadiene rubber, and the effect is not enough for isoprene-based rubbers such as natural rubber, highly purified natural rubber, isoprene rubber, and epoxidized natural rubber.
Tire rubber compositions generally contain isoprene-based rubbers together with styrene butadiene rubber or butadiene rubber. In particular, rubber compositions used for heavy-duty tires, which are required to have excellent rubber strength, mainly contain isoprene-based rubbers. Hence, effective techniques for improving the fuel economy of isoprene-based rubbers are desired.
Conventionally, sidewalls of pneumatic tires are formed from rubber compositions containing high unsaturation rubber such as natural rubber or isoprene rubber which has a high double-bond content in the main chain. Double bonds in such high unsaturation rubber molecules tend to react with ozone to cause depolymerization. If tires are left in a place exposed to strong UV light or in an atmosphere having a high ozone concentration in the air, cracking, so-called ozone cracking is caused. To avoid this phenomenon, rubber compositions for sidewalls contain ozone cracking inhibitors such as amine antioxidants or waxes.
Pneumatic tires including sidewalls formed from such rubber compositions containing ozone cracking inhibitors, however, have a dusty appearance due to blooming of the ozone cracking inhibitors on the sidewall surface during storage. In addition, amine antioxidants tend to turn their color to brown on exposure to the air or sunlight, and therefore the amine antioxidants migrated to the sidewall surface due to blooming discolor the sidewalls, lowering the commercial value of the tires. On the other hand, if the amount of antioxidant is reduced in order to reduce discoloration, cracks are formed earlier to lower the durability.
Ethylene-propylene-diene copolymers (EPDM) have fewer double bonds in the molecule, and are less likely to be attacked by oxygen, ozone, and the like. Accordingly, use of the EPDM in a sidewall provides a pneumatic tire capable of avoiding the occurrence of cracks on the tire surface to improve the durability, and also reducing the deterioration in appearance due to discoloration and the like (see Patent Literature 2, for example).
In cases where EPDM and a diene rubber are used in combination, however, a reinforcing filler such as carbon black or silica is dispersed preferentially in the diene rubber, and thus the reinforcement in the EPDM portion is lowered. In addition, since the reinforcing filler is concentrated in the diene rubber portion, more heat build-up tends to occur, that is, tan δ increases and therefore the fuel economy (the performance in terms of rolling resistance) is disadvantageously deteriorated. Thus, improvement in weather resistance, flex crack growth resistance and fuel economy in a balanced manner is needed.